1. Field of the Invention
The present invention relates to an ion generation device for generating ions from gas or vapor by plasma, an ion irradiation device, and a method of manufacturing a semiconductor device.
2. Description of the Related Art
Ion implantation has recently been employed as a method of forming a pn junction by adding impurities such as boron (B) and phosphorus (P) to a semiconductor substrate. The ion implantation makes it possible to accurately control the concentration and depth of impurities to be diffused into a target spot.
FIG. 1 is a block diagram illustrating the constitution of a prior art ion irradiation device. The constitution will now be described in brief.
In the prior art ion irradiation device, an ion source chamber 1 generates ions. The ions are drawn by drawing electrodes 2 and their mass is analyzed by a separating electromagnet 3. Then the ions are completely separated by slits 4 and accelerated by accelerators 5 to the final energy. A beam of the ions is converged on the face of a sample 12 by a quadrupole lens and scanned by scanning electrodes 7 and 8 such that it is uniformly distributed onto the entire face of the sample 12. Deflection electrodes 9 are designed to deflect the ion beam in order to eliminate uncharged particles caused by collision with residual gas.
The ion source chamber 1 is the heart of the ion irradiation device (including an ion generation device). As the chamber 1, there are a Freeman type ion source chamber using thermoelectrodes, a Bernas type ion source chamber, and a microwave type ion source chamber using magnetrons.
FIG. 2 is a cross-sectional view of a Freeman type ion source chamber 23. According to this chamber, plasma is generated by emitting thermoelectrons from a bar-shaped filament 22, an electric field is generated in parallel to the filament 22 by an electromagnet 21, a rotating field is caused by filament current, and electrons are complicatedly moved in the chamber 23 by a reflector 25, thereby improving in efficiency in ionization. The ions generated by the chamber 23 pass through a slit and are guided in a direction perpendicular to the filament 22.
FIG. 3 is a cross-sectional view of a Bernas type arc chamber 31 containing molybdenum (Mo) as the main ingredient. The chamber 31 includes a tungsten (W) filament 37 and its opposing electrode 34. The chamber is supplied with BF.sub.3 gas from a gas line 32 and emits thermoelectrons from the filament 37, with the result that the BF.sub.3 gas is discharged and ionized.
FIG. 4 is a cross-sectional view of a microwave ion source chamber. In this chamber, plasma is generated in a discharge box 43 using a microwave caused by a magnetron 41. Since the chamber has no filaments, its lifetime is not shortened even by the use of reactive gas or it is not contaminated with alkali metal. However, metal as well as ions is extracted from the chamber and attracted to the surface of a drawing electrode 44; therefore, a desired voltage cannot be applied or the metal or ions may reach a sample to contaminate it.
To resolve the above problems, an ion source chamber for applying carbon (C) onto the inner wall of the chamber containing molybdenum (Mo) has been manufactured experimentally. Gases such as BF.sub.3 and PF.sub.3 has been conventionally employed in the above conventional devices. These gases are ionized in the chamber to be implanted into source and drain regions serving as samples. In a compound semiconductor device, too, silicon (Si) is ion-implanted and, in this case, SiF.sub.4 is used as source gas.
Ion implantation of germanium (Ge) has recently been studied in order to render a semiconductor substrate serving as a sample amorphous. In the Ge ion implantation, GeH.sub.4 gas (germane gas) has been employed; however, it reacts upon oxygen drastically, which causes security problems. Thus, GeF.sub.4 is nowadays used as source gas.
If the above-described Freeman type chamber is used for a long time, the filament is increased in resistance and finally disconnected, with the result that no ions can be generated. Moreover, the chamber is not discharged because of conduction of the chamber and filament to be insulated by a supporting insulation member.
According to the foregoing Bernas type arc chamber, the molybdenum (Mo) on the inner wall of the chamber is etched by fluorine ions, radicals or the like produced by ionization of BF.sub.3, and the molybdenum (Mo) is etched and removed from the inner wall. The removed molybdenum absorbs on the tungsten filament 37 and electrode 34 and grown by thermal reaction into an alloy of W and Mo. In this case, the thermoelectrons are prevented from being emitted from the alloy region of the filament 37 and supplied only to that region thereof on which no molybdenum (Mo) absorbs, with the result that the filament 37 is locally used up and finally disconnected.
The disconnection of a tungsten filament used for about 20 hours will be explained referring to FIGS. 5A to 5C. Of these figures, FIG. 5A shows an unused tungsten filament, FIG. 5B does the surface of the tungsten filament when it is used for three days, and FIG. 5C is a graph showing the results of observation and composition analysis of a disconnected region of the filament. As shown in FIG. 5B, a foreign substance is included in a region of the filament where no disconnection occurs. It is turned out from the composition analysis that the foreign substance is an alloy of the tungsten (W) filament and molybdenum (Mo) absorbing thereon. In contrast, it is apparent from FIG. 6B that no foreign substance is present in a disconnected region of the filament and the filament is finely formed. In the region containing the foreign substance, the composition ratio of W to Mo is 26 wt % to 74 wt %, that is, the principal ingredient of the substance is molybdenum, while in the region containing no foreign substance, the composition ratio of W to Mo is 87 wt % to 13 wt %.
Furthermore, the microwave ion source chamber has the drawback wherein carbon (C) is removed from the surface of the chamber and absorbs on the filament or a sample is contaminated with the carbon.
In the prior art ion source chambers described above, a filament is disconnected or a supporting insulation member deteriorates in insulation properties to shorten the lifetime thereof, a semiconductor substrate is contaminated, and the like.